A Cs based secondary-ion mass spectrometry (SIMS) instrument has been constructed and coupled to a 3 MV tandem accelerator to produce an accelerator mass spectrometry system for impurity detection in materials. Exposed surfaces of the ion optical elements and sample holder hardware are fabricated from semiconductor grade Si to reduce sputter contamination of samples. The mass analyzed Cs beam generates negative secondary ions from the sample which are injected into the accelerator. Passage of the ions through the accelerator causes dissociation of the molecular interferences, so the system represents a truly "atomic" mass spectrometer. Quantification of the system is similar to SIMS, and sub parts per 10(9) detection limits of most transition metals have been demonstrated.